Enigmatic organisms preserved in early Ordovician macroborings, western Utah, USA
Macroborings in the Lower Ordovician Fillmore Formation, western Utah, USA, occasionally contain fossil remains of enigmatic organisms. In the most complete specimens a common morphology can be observed. The calcified body wall of the animal is vase-shape
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stract. Macroborings in the Lower Ordovician Fillmore Formation, western Utah, USA, occasionally contain fossil remains of enigmatic organisms. In the most complete specimens a common morphology can be observed. The calcified body wall of the animal is vase-shaped, mimicking the shape of the boring itself. An ovoid body leads up to a neck that contains either a single or double cylinder near the aperture of the boring. The incomplete preservation of the specimens is not sufficient to identify the biological affinity of the organism at this time, but a review of potential groups is warranted. While such groups as barnacles, bivalves, mitrates, and a host of worm-like forms are potential boring inhabitants, none fit what is known of the morphology of the specimens from Utah. Regardless, recognition and future identification of these animals will lead to a greater understanding of complex hardground trophic systems during the Ordovician Bioerosion Revolution. Keywords. Bioerosion, macroboring, hardground, Ordovician Bioerosion Revolution, Fillmore Formation
Introduction No body fossils representing organisms that are obviously capable of macroboring behavior have been found in rocks prior to the Pennsylvanian, and macroborings themselves are not abundant in rocks earlier than that age. Macroborings identified as Gastrochaenolites Leymerie, 1842 occur in the Lower Ordovician Fillmore Formation at Skull Rock Pass (SRP), western Utah, USA (Ekdale et al. 2002; Benner et al. 2004). Several specimens of these macroborings contain the presumed body fossils of enigmatic organisms. This is only the second known occurrence of an animal fossilized inside a pre-Pennsylvanian macroboring (Wilson and Palmer 1998). Because of the physically and chemically erosive environments in which hardground dwellers live, it is unlikely that a borer without a thickly shelled body would M. Wisshak, L. Tapanila (eds.), Current Developments in Bioerosion. Erlangen Earth Conference Series, doi: 10.1007/978-3-540-77598-0_3, © Springer-Verlag Berlin Heidelberg 2008
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survive. While it is unknown whether the organism was responsible for excavating the boring or was a secondary inhabitant, it is clear that these specimens represent an important source of new information regarding hard substrate infaunalization during the Ordovician Bioerosion Revolution (Wilson and Palmer 2006).
Fig. 1 Location and stratigraphy of the study area in the Fillmore Formation. A Pyramid Section locality at Skull Rock Pass, Utah. B Generalized stratigraphy of the Lower Ordovician Fillmore Formation showing the location of the Pyramid Section (black bar). Modified from Hintze (1973)
Enigmatic Ordovician macroborings
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Geological setting Macroborings containing the enigmatic organisms discussed in this paper occur in hardground surfaces in the Lower Ordovician (Ibexian) Fillmore Formation at SRP in Millard County, Utah (Fig. 1A). At the study locality, nearly 100 m of limestone and shale are exposed in outcrop, arranged in meter-scale shallowin
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